8315548: G1: Document why VM_G1CollectForAllocation::doit() may allocate without completing a GC

Reviewed-by: tschatzl

src/hotspot/share/gc/g1/g1VMOperations.cpp

@@ -127,6 +127,10 @@ void VM_G1CollectForAllocation::doit() {
 
   if (_word_size > 0) {
     // An allocation has been requested. So, try to do that first.
+    // During the execution of this VM operation, there may have been a concurrent active
+    // GCLocker, potentially leading to expansion of the Eden space by other mutators.
+    // If the Eden space were expanded, this allocation request might succeed without
+    // the need for triggering a garbage collection.
     _result = g1h->attempt_allocation_at_safepoint(_word_size,
                                                    false /* expect_null_cur_alloc_region */);
     if (_result != nullptr) {